1. Field of the Invention
This invention relates to compositions for encapsulating electronic components and, in particular, to compositions which contain ultra pure, fused silica fillers having low concentrations of radioactive contaminants.
2. Description of the Prior Art
Quartz, silica, and fused silica in powder form have been used for many years as fillers for encapsulating electronic components, such as, semiconductor memory devices. See, for example, Tuller et al., U.S. Pat. No. 4,042,550, and Rosler et al., U.S. Pat. No. 4,287,105. These fillers provide strength and low thermal expansion to the component package and are chemically inert.
As electronic components and, in particular, solid state memory chips, have become more complex and larger in capacity, they have become more sensitive to radioactive particles, in particular, alpha particles. See A. Robinson, "Problems with Ultraminiaturized Transistors", Science, 208:1246-1249, June 13, 1980; and W. Twaddell, "Upcoming High-Density-RAM Designs Must Fend Off Alpha-Particle Assaults," EDN, Nov. 20, 1980, pages 53-54.
For example, in the case of high density random access memories (RAMs), the passage of an alpha particle through the component can cause "soft" errors by changing the state of individual memory cells. The presence of radioactive elements in the filler materials used in encapsulating compositions for electronic components has been identified as a major source of such alpha particles. In particular, the uranium and thorium levels in existing silica fillers have been identified as significant sources of alpha particles.
Although this problem has been recognized for many years (see Robinson, supra, and Twaddell, supra), it has proved to be difficult to lower the levels of uranium and thorium in existing silica fillers. In particular, it has only been possible to achieve uranium levels on the order of 0.5 ppb under laboratory conditions, and then only for small samples at high cost.
The use of sol-gel techniques to produce fused silica articles, such as optical waveguides, has been considered in the past. In particular, the use of silicon-containing organic compounds, such as, tetraethylorthosilicate (TEOS) for such purposes has been disclosed. See Matsuyama et al., UK Patent Application No. GB No. 2,041,913.
The preparation of fused silica particles from a gel formed from fumed silica has been also been disclosed. Specifically, Bihuniak et al., U.S. Pat. Nos. 4,042,361, and 4,200,445, describe processes for densifying fumed silica and other fumed metal oxides by forming a sol, drying the sol to form fragments, and densifying the fragments by calcining them at 1150.degree.-1500.degree. C. Thereafter, the densified material can be milled, e.g., to an 8 to 10 micron average particle size.
Because it employs fumed silica, the Bihuniak et al. process is more difficult to perform than the process of the present invention. For example, it is relatively difficult to form gels from fumed silica, and as acknowledged in the Bihuniak et al. patents, once formed, gels made from fumed silica tend to break up into large chunks, rather than small particles. Further, extensive pollution abatement equipment is required to produce fumed silica since such production involves the creation of hydrochloric acid.
In addition, densified silica particles made from fumed silica generally have higher impurity levels than densified silica particles prepared in accordance with the present invention. These higher impurity levels are due in part to the fact that impurities, including radioactive impurities, are introduced into the silica during the fuming process.
The higher impurity levels also arise from the fact that densification of particles made from fumed silica gels requires higher temperatures than densification of particles formed from gels made from silicon-containing organic compounds, i.e., densification of particles made from fumed silica gels require temperatures above, rather than below, 1150.degree. C. Such higher temperatures generally mean that metal-containing furnaces must be used to perform the densification. The use of such furnaces, in turn, means that the silica particles will be exposed to and thus will pick up contaminants released from the walls of the hot furnace. In addition to the purity problem, the need to generate higher temperatures to achieve densification is in general undesirable.